Sheet metal tuning fork



Aug. 30, 1966 D. R. DAILEY swam METAL TUNING FORK Filed March 26, 1965 RY ms t MM R D .L A N o D 6 2 5 m0. T 1 T 2 1X 3 6 2 ATTORNEYS UnitedStates Patent 3,269,249 SHEET METAL TUNING FORK Donald R. Dailey,Leesburg, Va., assignor to Melpar, Inc., Falls Church, Va., 'acorporation of Delaware Filed Mar. 26, 1965, Ser. No. 442,955 9 Claims.(Cl. 84-457) The present invention relates generally to sheet metaltuning forks .and more particularly to a fork having its tines mountedso they vibrate in the plane of the sheet.

Prior sheet metal tuning forks have been constructed by bending a stripof sheet metal into the shape of a U. The fork tines have been arrangedso the thickness of the sheet vibrates in planes at right angles to thedirection of forces that cause oscillation. Thus, the width of the sheetis subjected to the driving forces derived from a source of oscillatoryenergy. A typical example of such a prior art tuning fork is shown inUS. Patent 3,106,124, issued to William I. Asten.

Stability of the oscillations deriving from a sheet metal tuning forkassembly is related to the mass ratio of the fork to the base assembly,such that a relatively high ratio can result in oscillations at afrequency different from the designed natural fork frequency. To achievestability, it is incumbent upon tuning fork designers to provide asupport having a mass at least equal to, and preferably considerablygreater than, the mass of the fork. The most preferred manner forachieving this result for a modern, miniature device is to reduce thefork mass as much as possible so that the weight and size of theassembly is minimized. Since, however, fork mass and dimensions are afunction of frequency, a rigidly prescribed parameter, an anomoly isseemingly reached between the parameters governing minimum fork assemblysize and frequency.

I have found that this problem can be overcome, to a large extent, bystamping the fork as a sheet having a U shape and mounting the resultingstructure so that the tines vibrate in the plane of the sheet; Thenatural frequency of the fork of the present invention is determined,primarily, by the time length and Width, with the thickness dimensionhaving only a relatively small effect resulting from changes due to itsspring tension properties. Actually, the sheet thickness need only begreat enough to provide sufiicient rigidity to the tines whereby forkvibrations and/ or bending in planes other than those in which the sheetlie are precluded.

Because the width of the stamped tines in their direction of vibrationcan be made substantial according to the present invention, the forkmass necessary to achieve a particular frequency is reduced incomparison to the prior art devices. Because of the reduced fork mass,it is possible to provide a base of lower mass and still achievestability. In consequence, the weight and size of the entire unit isreduced over what the prior art achieves for a unit having the sameoutput characteristics.

By stamping the tines from sheet metal, the cost of fabrication isreduced considerably from the prior art, bent sheet metal fork. Thestamping of multiple forks can be accomplished in a facile andsimultaneous manner in comparison to the somewhat tedious and individualbending operations of the prior art.

It is, accordingly, an object of the present invention to provide a newand improved sheet metal tuning fork.

It is another object of the present invention to provide a sheet metaltuning fork in which the tines are stamped in a U-shape.

A further object of the present invention is to provide a sheet metaltuning fork wherein the tines vibrate in the plane of the sheet.

Still another object of the invention is to provide a sheet metal tuningfork wherein the width of the tines 3,269,249 Patented August 30, 1966in the plane of their vibration is appreciably greater than thethickness of the sheet metal.

An additional object of the invention is to provide a sheet metal tuningfork wherein sheet thickness has no substantial effect on frequency butis relied upon solely for maintaining fork rigidity.

Yet a further object of the invention is to provide a new and improvedsheet metal tuning fork that is less expensive, has a smaller mass andoccupies less space than prior art devices. I

A still further object of the invention is to provide a new and improvedtuning fork assembly having low mass tines mounted on a relativelysmall, low mass support, wherein the fork to support mass ratio issufliciently low to enable stable oscillations to be derived.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIGURE 1 is a perspective view of the sheet metal fork and supportaccording to one embodiment of the invention;

FIGURE 2 is a side sectional view of the entire assembly, taken throughthe lines 2--2 of FIGURE 3.

FIGURE 3 is a front sectional view taken through the lines 3-3 of FIGURE2;

FIGURE 4 is a top sectional view taken through th lines 44 of FIGURE 3;and

FIGURE 5 is a top view illustrating the positions of the tines at twodifferent times during a period of oscillation.

Reference'is now made specifically to the figures wherein U-shaped sheetmetal fork 11, stamped from a highly permeable magnetic material, suchas NI-SPAN-C, is mounted toward one edge of a rectangular support 12that has sufficient thickness to provide adequate support for the fork.Pork '11 is bonded to support 12 by solder of welded joint 13 at thecenter of its leg 14 that joins tines 15 and 16 together. Support 12 isfixedly mounted on plate 17 by two screws 18 that thread into bores 19.

The complete fork-support assembly is designed so that fork mass isconsiderably less than the support mass to achieve stable oscillations.

Cylindrical, drive and pick up coils 21 and 22, respectively, havingpermanent magnets 23 and 24 coincident with their axes, are fixedlypositioned so their aligned longitudinal axes extend in a plane at rightangles to the plane in which fork 11 lies and is substantiallycoincident with the plane containing the fork center line. Windings 21and 22 extend almost to the face of fork 11, but no farther since theirouter circumferences extend beyond the inner edges of tines 15 and 16.

Magnets 23, 24, coils 21, 22, as well as the electronic circuitsnecessary to provide a completely transistorized oscillator, areencapsulated or potted into a unitary, sol-id body 30 with a hightemperature, stable plastic. In the encapsulating operation, mountingplate 17 for base 12 and header 20, both of which are metallic, arebonded to either end of body 30. The power and signal leads 31 for thetransistor oscillator extend through insulated segment 32 in header 20.I

By utilizing the tuning fork of the present invention, it is possible tomount the entire tuning fork oscillator in a sleeve-like metal (magneticor non-magnetic) case 25 having approximate exterior dimensions of 1.5"length, 4-" width and breadth. Of this space, the tuning fork tines andbase, driving and pickup coil magnet structures occupy an approximatevolume of 1%" long, wide and A" in breadth. The total assembly, withleads attached, weighs less than one ounce.

The entire tuning fork assembly encapsulated in body 30 when slid intocontainer 25 is such that fork 11 and the exposed ends of magnets 23 and24 are proximate to container wall 33. The unit possesses an optimumfork to support mass ratio, as a result of tines 15 and 16 beingarranged so that the sheet metal thickness, denoted by reference numeralC, extends in the same direction as the axes of coils 21, 22. Inaddition, the construction em ployed wherein fork 11 is disposed alongone wall of container 25, enables coils 21, 22 and magnets 23, 24 to beof maximum size, so that an output voltage of large amplitude isderived.

To provide maximum magnetic field coupling between windings 21, 22 andvibrating tines 15, 16, permanent bar magnets 23 and 24 are aligned withthe winding axes and extend between the tines, almost to the wall ofmetal container 25. Magnets 23 and 24 are polarized so that the northpole of the former and the south pole of the latter are proximate fork11. Hence, the DC. magnetic reluctance path can be assumed asoriginating at the surface of magnet 24, closest to the free ends oftines 15, 16. The path continues, in parallel, through tines 15', 16 tothe location of magnet 23 where it combines at the south pole thereof,and goes through that magnet to its north pole. The permanent magneticfield proceeds through permeable container 25 to a position opposite thesouth pole of magnet 24. From there, it jumps the air gap to the southpole of magnet 24 and proceeds through that magnet to its north pole.

The magnetic field flux variations deriving from coil 21 result invibration of tines 15 and 16 with opposed velocities in the plane of thesheet. The rest position of tines 15 and 16 is shown in the top view ofFIGURE by the solid line rectangles denoted by reference numeral 26while the most extreme inward position of the tines during a cycle ofoscillation is illustrated by the dashed line rectangles denoted byreference numeral 27. It is noted that front edges 35 of the tines arealways in the same plane during the entire oscillating cycle.

The natural resonant frequency of fork 11 is determined, to a largeextent, by the width, B, and length, A, of tines 15 and 16. The forkfrequency isnot particularly aflfected by the tine thickness, C, whichneed only be great enough to provide sufiicient rigidity to maintain thefork as a high Q, single frequency mechanical resonator.

Hence, the starting material for fork 11 can be a typical piece of sheetmetal that has a thickness in the range between 0.020 inch and 0.050inch. The lower limit for the thickness of the sheets is the minimumthickness that provides the required rigidity, while the upper limit isset :by the maximum fork to support ratio necessary to provide stableoscillations at the designed frequency.

In a typical embodiment actually constructed, the width, B, of stampedtines 15 and 16 was on the order of 0.100", their length, A, wasapproximately 0.75, sheet thickness was 0.025" and the distance betweenthe outer tines 15 and 16 was approximately The output frequencyprovided by this unit was on the order of 1400 c.p.s. with a Q ofapproximately 5000.

While I have described and illustrated one specific embodiment of myinvention, it will be clear that variation of the details ofconstruction which are specifically illustrated and described may beresorted to without departing from the true spirit and scope of theinvention as defined in the appended claims.

I claim:

1. An electro-mechanical resonator assembly comprising a magneticallypermeable, U-shaped, sheet metal tuning fork having a pair of tinesjoined by a leg between them, the thickness of said U beingsubstantially equal to the thickness of said sheet, a support for saidfork fixedly mounted to said leg so said tines vibrate in the plane ofthe sheet relative to the mounting position and electromagnet meansbeing positioned to drive said tines so they vibrate in the plane of thesheet.

2. An electro-mechan-ical resonator assembly comprising a magneticallypermeable, U-shaped sheet metal tuning fork having a pair of tinesjoined by a leg between them, the thickness of said U being sufiicientto provide rigidity to the fork and such that it has no substantialeffect on the natural resonant fork frequency, said frequency beingdetermined by the length and width of said tines, a support for saidfork fixedly mounted to said leg so said tines vibrate in the plane ofthe sheet relative to the mounting position, and electromagnet meansbeing positioned to drive said tines so they vibrate in the plane of thesheet.

3. The assembly of claim 2 wherein said thickness is at least 0.020".

4. The assembly of claim 2 wherein said thickness is in the rangebetween 0.020 and 0.050".

5. An electro-mechanical resonator assembly comprising a magneticallypermeable, stamped, U-shaped, sheet metal tuning fork having a pair oftines joined by a leg between them, the thickness of said U beingsubstantially equal to the thickness of said sheet, a support for saidfork fixedly mounted to said leg so said tines vibrate in the plane ofthe sheet relative to the mounting position and electromagnet meanspositioned to drive said tines so they vibrate in the plane of thesheet.

6. An electro-mechanical resonator assembly comprising a magneticallypermeable, stamped, U-shaped, sheet metal tuning fork having a pair oftines joined by a leg between them, the thickness of said U beingsuflicient to provide rigidity to the fork and such that it has nosubstantial effect on the natural resonant fork frequency, saidfrequency being determined by the length and width of said tines, asupport for said fork fixedly mounted to said leg so said tines vibratein the plane of the sheet relative to the mounting position, andelectromagnet means positioned to drive said tines so they vibrate inthe plane of the sheet.

7. An electro-mechanical resonator assembly comprising a magneticallypermeable, stamped, U-shaped sheet metal tuning fork having a pair oftines joined by a leg between them, the thickness of said U beingsufiicient to provide rigidity to the fork and such that it has nosubstantial effect on the natural resonant fork frequency, saidfrequency being determine-d by the length and width of said tines, asupport for said fork fixedly mounted to said leg so said tines vibratein the plane of the sheet relative to the mounting position, said forkbeing mounted at one edge of said support, a mounting plate secured tosaid support at the edge opposite to said one edge, drive and pick upcoils for said fork positioned to have their longitudinal axes extendingsubstantially along the plane containing the center line between thetines.

8. The assembly of claim 7 wherein the outer circumferences of saidcoils extend beyond the inner edges of said tines, and said coils extendproximate but not to the face of the fork, a permanent bar magnet lyingalong the longitudinal axis of each coil extending into said coil andbetween said tines.

9. The assembly of claim 8 further including a container having a pairof parallel walls, said fork and the ends of said magnet extendingbetween said tines being disposed in proximity to one wall of saidcontainer.

References Cited by the Examiner UNITED STATES PATENTS 2,994,241 8/1961Gibbs 84-409 3,083,607 4/ 1963 Reifel 84-409 3,106,124 10/1963 Asten84457 3,122,047 2/1964 Jones et al. 84-409 3,167,905 2/1965 Hetzel310-25 X RICHARD B. WILKINSON, Primary Examiner.

1. AN ELCTRO-MECHANICAL RESONATOR ASSEMBLY COMPRISING A MAGNETICALLYPERMEABLE, U-SHAPED, SHEET METAL TUNING FORK HAVING A PAIR OF TINESJOINED BY A LEG BETWEEN THEM, THE THICKNESS OF SAID U BEINGSUBSTANTIALLY EQUAL TO THE THICKNESS OF SAID SHEET, A SUPPORT FOR SAIDFORK FIXEDLY MOUNTED TO SAID LEG SO SAID TINES VIBRATE IN THE PLANE OFTHE SHEET RELATIVE TO THE MOUNTING POSITION AND ELECTROMAGNET MEANSBEING POSITIONED TO DRIVE SAID TINES SO THEY VIBRATE IN THE PLANE OF THESHEET.